Radioactivity spectrometry in sample analysis



RAnIoAcTIvm sPEcTRom-:TRY 1u SAMPLE ANALYSIS Filed April 9, 1962 Dec. 17, 1963 L. E. PACKARD 6 Sheets-Sheet l INVENTOR. LYLE E. PAcKARQ Dec. 17, 1963 L, E, PACKARD 3393?@335 RADIOACTIVITY SPECTROIETRY IN SAMPLE ANALYSIS Filed April 9, 1962 6 Sheets-Sheet 2 HIGH VOLTAGE SUPPLY Dec. 17, i963 l., E. PACKARD RADIOACTIVITY SPECTROMETRY IN SAMPLE ANALYSIS Filed April 9, 1962 6 Sheets-Sheet 3 Vx PULSE HEIGHT CPM PULSE HEIGHT (Fm-:o GAIN) l PULSE HEIGHT CPM INVENTOR LYLE @www agen? vrvs 6 Sheets-Sheet 4 L. E. PACKARD RADIOACTIVITY SPECTROMETRY IN SAMPLE ANALYSIS Filed April 9, 1962 RADIoAcTIvITY SPECTROMETRY 1N SAMPLE ANALYSIS Filed April 9, 1962 Dec. i7, 1963 E. PACKARD 6 Sheets-Sheet 5 PULSE HEIGHT CPM PULSE HEIGHT CHANNEL I AND CHANNEL [I -HIGH GNN INVENTOR LYLE E. PAcKAnn BY ijlwl,

Affvs Dec. 17, 1963 1 E. PACKARD S 4,835

RADIOACTIVI'I'Y SPECTROMETRY IN SAMPLE ANALYSIS Filed April 9. 1962 6 Sheetsf-Shevat 6 CPM PULSE HEIGHT (enamel. l1

CPM

PuLsE HEIGHT (CHANNEL I INVENTOR. LYLE E. PACKARD aliases Patented Dec. i?, i963 Filed Apr. 9, 1962, Ser. N0. 186,066 14 Claims. (Cl. 250-71.5)

The present invention relates in general to a method and apparatus for detecting and measuring radioactivity emanating from a source and, more particularly, to spectrometry for detecting the energy spectra, and measuring certain bands of thc spectra, of radioactive isotopes.

ln its primary aspect, the invention is concerned with.

radioactivity detecting methods and apparatus for measuring the activity level oi radioactive sources which comprise two or more diterent radioactive isotopes; or dit'- ierent sources (or a changing source) which respectively contain one or more of a plurality of difierentisotopes.

lt is a general aim of the present invention to provide an improved radioactivity spectrometry method and apparatus characterized by simultaneous and independent detection and counting of radioactivity from two or more isotopes irrespective of whether such isotopes are present in composite sources, or individually present in separate sources. yet wherein the count for each isotope is highly etiicient and statistically accurate. While not so limited in its application, the invention will nd especially advantageous use in measuring the activity levels of two different isotopes in a single sample (commonly referred to as a double-label sample), and in measuring the activity levels of isotopes in ditierent samples, each sample including at least one of two different isotopes (commonly referred to as mixed samples).

A related object of the invention is to provide a radioactivity spectrometry method and apparatus for distinguishing between and accurately measuring two or more diilerent radioactive isotopes, wherein one presetting or adjustment makes possible simultaneous measuring of the different isotopes quickly and with high statistical ac-l curacy, und yet characterized by thc elimination of readjustment and duplicated counts for each of the isotopes involved. As a consequence of attaining this objective, the elapsed time heretofore required for changing samples and "reading out" the counts in plural-label and mixed sample radiation measurements is materially reduced and the procedure greatly simplified.

Another object of the invention is to provide a highly versatile radioactivity detecting method andapparatus characterized by its ability to be used with any one of a variety of detectors,` thus affording measurements of a wide range of double-label samples or mixed samples containing radioactive isotopes which may, for example, emit gamma rays, alpha particles or beta particles.

An important object of the invention is to provide radioactivity detecting apparatus having two or more separate and independent channels which permit either simultaneous or sequential counting of two or more different radioactive isotopes, yet-wherein each channel may he adjusted to count emissions from a particular isotope at optimum counting conditions, i.e., with high statistical accuracy in relatively short counting periods, and with substantial immunity from drifting in characteristics of amplifiers and other electronic devices.

lt is a coordinate and more specific object of the invention to provide a multiple-channel scintillation spectrometer in which radioactivity emanating from different isotopes may be accurately counted in a single operation by analyzing the pulse heights of signals produced by each emission, yet wherein pulse height analyzing in each channel is entirely independent and wherein the ampli- 2 tude of pulses in one or more channels may be selectively adjusted or attenuated, thus permitting each channel to count a different given isotope at substantially optimum conditions while all other isotopes contribute only to a small degree to the count in that channel.

Still another object of the invention is to achieve such improved multiple-channel radioactivity spectrometry through the use of conventional amplifying circuits, yet wherein the inherent operating characteristics or limitations such as noise and saturation of those circuits do not impose limitations on the accuracy of measurements based upon pulse-height analysis of multiple isotopes.

Other objects and advantages of the invention will become apparent as the following description proceeds, taken in conjunction with the accompanying drawings, in which:

FIGURE l is a generalized diagrammatic block-andline representation of a prior radioactivity spectrometer;

FIG. 2 is a diagrammatic block-and-line representation `of a` conventional coincidence type liquid scintillation spectrometer particularly suited for detecting and measuring the less penetrating alpha or beta particles emanating from radioactive isotopes;

FIG. 3 is a schematic diagram, partly in block-andline form, ot' exemplary photomultipliers, together with an adjustable high voltage supply therefor;

FIG. 4 is a graphic representation of a typical set' of spectral curves for a given beta emitting isotope with each curve representing a spectrum of voltage pulse height derived from decay events of that isotope at a different high voltage setting of the power supply;

FIG. 5 is a graphic representation of a typical pulse height spectrum characteristic of a beta emitting isotope `and illustrating particularly the principle of balancepoint" operation;

FIG. 6 is a graphic representation of the random distribution of pulses of varying amplitude which are derivcd from decay events produced by two ditferent beta emitting isotopes present in a single sample or source;

FIG. 7 depicts in graphic form the two spectral distributions of the pulses shown in FIG. 6, each spectrum representing pulses originating from one of two isotopes;

FIG. 8 is a graph similar to FIG. 7 illustrating in addition a typical curve representing the distribution of spurious background pulses and showing also the minimum and maximum pulse heights which can be accurately discriminated and counted;

FIG. 9 is a diagrammatic block-and-line representation of a multiple-channel scintillation spectrometer embodyingthe features of the present invention;

FIG. lO is a schematic wiring diagram of an exemplary attenuator or gain control device for varying the gain of one analyzing channel with respect to another analyzing channel;

FIG. ll is a schematic wiring diagram illustrating the details of an exemplary discriminator circuit;

FIG. l2 is a graphic representation of the pulse spectra for two isotopes at the channel I and. channel ll amplifier outputs with equal gains and at a given setting of the variable high voltage supply;

FIG. t3 is a graph similar to FIG. l2 and illustrating the pulse spectra at a second and higher setting of the variable high voltage supply; and

FIGS. 14 and 14A are graphs respectively illustrating the pulse spectra for the two isotopes at the channel I and channel II amplifier outputs, the two channels here having different gains.

While the invention is susceptible of various modifications and alternative forms. a specific embodiment thereof has been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular form disclosed, but, on the contrary. the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as expressed in the appended claims.

THE ENVIRONMENT OF THE INVENTION Before treating the present invention in detail, it will be helpful first to consider briefly the prior art background or environment. In radioactivity measurements, it is the most frequent objective to determine the rate at which decay events in an isotope present in a radioactive source occur, this rate generally being expressed as counts per unit time, eg., counts per minute. The quantity of a particular isotope present in a radioactive source is in general proportional to the rate of decay events produced by that isotope, such rate being termed the activity level of the source. As a generalization, the decay events or radiation emanations from a radioactive source are, for purposes of measurement or counting, converted into corresponding voltage pulses which can then be counted. The pulses may be counted for a predetermined time period, or the time necessary to receive a predetermined number of pulses may be measured. the ratio of counted pulses to the elapsed time period being indicative of the activity level. In some instances the voltage pulses may be fed to a direct-reading or recording rate meter which indicates the activity level.

(a) General Organization and Operation of Spectrometer The spectrometer illustrated diagrammatically at in FIG. l includes a radioactive source 22 disposed in operative relationship to a detector or proportional transducer 24. the latter serving to convert radiation from decay events within the source 22 into corresponding electrical signals. egt, voltage pulses. The transducer may be any one of a variety available in the art, such for example as a sodium iodide thalium activated) crystal for converting radioactive decay events into light flashes. A transducer comprised of such a scintillation crystal and a photomultiplier for converting light flashes into voltage pulses is suitable for detecting radiation from gammaemitting isotopes; but still other transducers constituting a source of pulses may be employed for different specific applications. Regardless of the particular type of delector and transducer employed, it should have a proportional characteristic, i.e., each voltage pulse will be substantially proportional to the energy of the decay event which produces it.

The voltage pulses so derived by the detector or transducer 24 are. however, of relatively low amplitude. It is impractical, if not impossible, to discriminate these pulses on the basis of differences in their amplitudes and to count or measure their rate of occurrence. Accordingly, the pulses from the detector 24 are first passed through a linear amplifier 28 and then supplied to a pulse height analyzer 29. Because the pulses received by the analyzer have been amplitied and thus occupy a fairly wide spectrum of amplitudes, the analyzer 28 may select only these pulses which lie above or below certain preselected amplitudes, and pass them to a nal indicating device such as a rate meter or a sealer, the latter being shown at 30. By adjusting the amplitude band or window of pulse amplitude passed by the analyzer 29, the background pulses (resulting either from spurious radiation, cosmic rays, or noise in the transducer) can be substantially reduced, so that the count received by the sealer is comprised principally of pulses resulting from the activity of the source being measured. The background pulses to a large extent have amplitudes that fall outside the acceptance band of the analyzer after the latter has been adjusted. The background count passed by the analyzer may, of course, be measured for a given time interval with no radioactive source present, and then subtracted from subsequent sealer counts received with radioactive sources present.

Because the pulses applied to the input of the analyzer 29 have amplitudes substantially proporitonal to the energy of corresponding decay events in the source 22, the analyzer may be adjusted successively to pass bands of pulse amplitudes, and the energy spectrum of the decay events in an isotope thus plotted. Once that spectrum has been measured, or is known, the analyzer 29 may be set to pass an amplitude band or window in which a fairly large proportion of all pulses derived from the isotope decay events are passed and counted, yet in which the background or spurious pulses passed to the sealer are relatively few. The proportion of pulses counted from source decay events to the total number of decay events in the source is termed the efficiency of counting. It is generally accepted that the optimum counting conditions are obtained, i.e., statistically accurate results produced with reasonably low total counts and in relatively short counting time periods, when the ratio E2/B is made substantially a maximum (where E represents the efficiency of counting pulses derived from an isotope, and B represents the background count).

(b) Liquid Schilf/lation and Coincidence Monitoring In spectrometry involving isotopes which produce radiation particles having relatively low penetrating power, and particularly with samples or sources of low activity levels, the detector or transducer may lpreferably comprise a solution of liquid scintillator into which the radioactive substance is added. Light flashes in this solution, resulting from decay events of the isotope, are transmitted to a photosensitive electrical device, preferably a photomultiplier tube. Because such photomultiplier tubes are to an undesirable degree prone to produce spurious voltage pulses due to dark noise current" which is primarily caused by thermally induced electron emission, it has been common practice to employ coincidence monitoring" in order to preclude counting of these spurious voltage pulses. Such a liquid scintillation spectrometer with ycoincidence monitoring, and intended primarily for work with alpha and beta radiation isotopes, is diagrammatically illustrated in FIG. 2.

Referring to FIG. 2. the radioactive source is shown at 32 as a sample of isotope-containing substance dissolved or suspended in a liquid seintiliator, the latter being in a container having light-transmissive walls. Aromatic ethers are commonly employed as solvents, although numerous other solvents are known in the art. Any one of numerous commercially obtainable .scintillators -or uorescent materials is also dissolved in the solvent for the purpose of converting the radiant energy resulting from a decay event (for example, an alpha or beta decay event) into light energy. Finally, the sample includes the radioactive material or isotope to be measured. The emission energy spectra of the different radioactive materials may vary greatly, each having a characteristic, known spectrum. A typical, but non-exhaustive list of isotopes which may be counted with the spectrometer of FIG. 2 is set forth below in Table I:

Isotope Halt Lite Max. Bota Isotope Half Lite M ax. Bota Energy, Energy, lllV HIGV.

0.018 Nul 0. 5S 0.15 Im... 060,031 (l. lT Cl, 0. Tl l). 2G 1.33 (l. 32 1.39 0. 46,0.29 l. Tl

ln working withv isotopes of the type exemplified in Table l, the light flashes in the scintillator solution are relatively weak, although proportional in intensity to the energy of the decay events which produce them. lt is for this reason that a very sensitive light-to-voltage transduccr, such as the relatively high gain photomultiplicr tube is employed, and the coincidence monitoring accomplished to reduce the effects of noise pulses therein.

Referring more specifically to FIG. 2, the spectrometer 3l there illustrated includes a pair of photomultiplicrs 24', 24 which are placed contiguous with or adjacent to the sample 32 and energized from a variable high voltage source 25. The photomultipliers 24' and 24" serve lhe respective functions of providing pulses to be analyzed and pulses to monitor or gate the first pulses when both photomultipliers simultaneouslyrespond. The outputs of the photomultipliers 24', 24", are respectively coupled to preampliiers 26', 26". ln order to 4reduce the number of thermal noise pulses in the photomultipliers, the sample 32, photomultipliers 24', 24" and preampliliers 26'. 26" are preferably all located within a cooled chamber or freezer. diagrammatically illustrated at 34. The

outputs of the preamplifiers 26 or 26" are respectively coupled to amplifiers 28' and 28" which in turn pass amplified pulses to pulse height analyzers 29' and 29". Each analyzer provides an input pulse or signal for a suitable logic circuit 35 which, for example, may simply be an AND gate. The output of the AND gate iscoupled directly to a sealer 30'.

lVhen a decay event (for example, a beta emission) occurs in the sample 32, a light scintillation occurs that is simultaneously detected by both photomultipliers 24', 24" and signal pulses proportional in amplitude to the energy of the decay event are simultaneously produced at the output of the amplifiers 28', 28". These I'pulses are then analyzed in pulse height analyzers 29' and 29", the former being adapted so that it may pass only a selected band of pulses. The analyzer 29", on the other hand, preferably is constructed and adjusted to pass all received pulses which exceed a predetermined low amplitude, and need not be restricted as to the upper amplitude of passed pulses. Only when the analyzers 29', 29" provide Coincident input pulses to the AND gate 35, does the latter produce an output pulse which is counted by the scalcr 30'. lf, on the other hand, coincident inputs are not present at the AND gate 3S, any pulse from the analyzer 29' is blocked from and not counted by the sealer 30'. Merely by way of example, it will be readily apparent that when a thermal pulse is generated in either one of the photomultipliers, coincident input signals will not be present at the AND gate 35 and hence no count will be recorded by the sealer 33' even if the thermal pulse is within the amplitude band or window being passed by the `associated analyzer.

(c) Typical Photomultiplicr Arrangement An exemplary circuit for and diagrammatic illustration of the photomultiplicrs 24' and 24" is shown in FIG. 3. The photomultiplicrs 24', 24" respectively include generally cylindrical, opaque envelopes 38', 38" having light transmissivc end walls 39', 39" disposed in proximity to the sample and scintillator 32. Photosensitive cathodes 40', 40" are respectively provided in the photomultipliers 24', 24" in proximity to the light transmissive end walls 39', 39". Each photomultiplier 24', 24" also includes a plurality of dynodes iT-51', 41"51, respectively, which are held at progressively higher potentials relative to their corresponding cathodes. To accomplish this, the cathode and dynode potentials are derived from a pair of voltage divider circuits 52', 52", respectively, including series resistors Rl'-Rll', Rl"-R1l" and parallel shunting capacitors (IY-Cil', C1"-Cl1". Each voltage divider circuit is connected in series with the high voltage supply 25, thc latter being provided with an adjustable switch arm 54 adapted to engage a selectable one of the terminals T0-T9 residing at progressively higher voltages.

Considering. for example, the photomultiplicr 24', the voltage drop uross resistor Rl' is applied between the photosensitive cathode 40' and the dynode 41', -while the voltage drop across resistance R2' is applied between dynodes 41' and 42'. The voltage drop across cach' succceding resistor is in turn applied between each suececding pair of dynodes. The arrangement is such that when a decay event, for example, a beta emission, occurs in the sample 32 it produces a light scintillation which is simultaneously detected by the photoscnsitive cathodes 40', 46" through the light transmissive end walls 39', 39". As the light rays impinge upon the photosensitive cathodes, electrons proportional in number to the energy of the light are emitted. The emitted electrons are attracted to the dynodes 41', 41" which are at a higher voltage, thus producing by bombardment still more electrons which are in turn attracted to the next higher potential dynodes 42', 42". This process continues until the electrons reach the anodes 51', 51" and thus cause current flow through and voltage pulses across load resistors 53', 53". These voltage pulses appear at the input of each of the preamplifiers 26', 26" (FIG. 2). i

The total gain produced by each of the photomultipliers 24', 24" can be varied by changing fthe magnitude of high voltage, Le., by shifting the switch arm 54 to a selected one of the terminals Til-T9 on the high voltage supply 25. Fine adjustment of the high voltage applied across the voltage divider networks 52', 52'l may be obtained by providing a :trimmer 55 which permits selective adjustment of the high voltage between the fixed voltages of adjacent terminals.

In order to simplify the ensuing discussion, isotope decay events will herein be referred to by way of example as beta decay events or beta emissions. It will be understood, however, ythat alpha and gamma radiation may be considered in the same way, even though there may be mono-energetic spectra involved in some instances.

(d) Spectral Distributions and Optimum Counting Conditions It is well known that beta-emitting isotopes produce decay events which individually involve energies spread over a fairly wide range or spectrum.. Each isotope has its own characteristic spectrum with a known maximum energy (see Table l, supra). A small proportion of the decay events have relatively high and low energies, and the majority have energies in the middle region between the upper and lower limits. For a given gain of the transducer, which forms a source of pulses proportionally related to the energies of the decay events, the amplitude spectrum of the pulses corresponds to the energy spectrum of decay events.

Referring next to FIG. 4 there is graphically illustrated a set of curves characteristic ofthe pulse height spectra ot a typical beta emitting isotope sample, each curve representing the distribution of pulse heights for a different setting of the tap 54 of the high voltage supply 25. Since the amplitudes of output pulses from the amplifiers 28', 28" (FlG. 2) are proportional to the energy of the beta emissions which caused the pulses, the abscissa of the graph shown in FIG. 4 is here calibrated in volts (pulse height), while the ordinate is calibrated in counts per minute (c.p.rn.). The factor of proportionality is, however, changed when thc high voltage applied to the photomultipliers is changed, but the total number of pulses from a sample having a given activity level remains constant. Thus, the areas under the several curves of FIG. 4 arc all substantially equal. For example, let it be considered that the curve S8 represents the pulse height spectrum for a given sample of an Isotope l at a given relatively low voltage applied to the photomultiplicrs, i.e., with the arm 54 (FIG. 3) set at terminal Tl of the high voltage supply 25. Over a given period of time Isotope I will undergo a plurality of decay events of varying encrgies. As is typical of bota emitting isotopespt'here will be a few decay cvcnts of minimum energy, producing very small pulses Vmm which approach zero amplitudes.

There will also be a few decay events of maximum energy, producing maximum amplitude pulses, Vmax. However, the greatest number of decay events will result in pulses of approximately the average amplitude Vavg.

lf the same sample is retained, but the tap 54 on the high voltage supply 25 is now moved to terminal T2 to increase the high voltage applied across the voltage divider circuits 52', 52, the amplitude of all the pulses emanating from the amplitier 28', 28 will correspondingly be multiplied. That is, the pulse height spectrum for Isotope I is now represented by the curve 59 having a few pulses of minimum amplitude Vmm, a few pulses of maximum amplitude ZVmX, and a large proportion of pulses of approximately average energy 2Vavg. However, since the total activity of the isotope, and the spectral distribution of the energies of decay events, remain unaffected by the increased gain, the area under the curve 59 remains the same as the area under the curve 5S. The curves 60, 61 and 62 shown in FIG. 4 are respectively representative of the pulse height spectra of a sample of Isotope I having a given activity, but for different photomultiplier gains when the tap 54 is set at terminals T4, T6 and T8. In each instance, the area under the curve which is representative of the total activity of the isotope sample remains unchanged. However, the amplitude of the pulses representative of each decay event are proportionately increased with corresponding increases in the photomultiplier gain.

lf it is now assumed that the vertical lines A and B (FIG. 4) respectively represent the minimum and maximum pulse amplitudes which are passed by the pulse height analyzer 29', it will be immediately apparent that no pulse will be counted while the high voltage supply 25 is set at terminal T1 (i.e., no portion of the area under curve 58 lies in the window defined by limits or base lines A and B). When the photomultiplier gain is doubled to produce the curve 59, a few of the higher amplitude pulses representative of decay events are of suicient amplitude to exceed the minimum base line A of the pulse height analyzer 29' and these will be passed therethrough. It will be observed that when the tap 54 is movedto terminal T4 (curve 60), the peak of the Spectral curve 60 is located approximately midway between the basp Klines A, B of the pulse height analyzer 29'. At this setting of the tap, a maximum area under the spectral curve lies within the particular window defined by the discrimination levels A and B and consequently the spectrometer is capable of counting decay events with high eticiency, yet with the exclusion of background or noise pulses which lie predominantly outside the window. As the gain is increased still further by moving the -tap progressively to terminals T5-T9, a progressively smaller area of the curve will be located between the window limits A, B, thus decreasing the eiciency of the count.

A real limitation imposed upon spectrometer apparatus resides in the inherent operating limitations or characteristics of the electronic equipment employed. For example, as those skilled in the art are aware, a given amplitier inherently has a saturation level beyond which amplilcation becomes nonlinear. Linear amplification of input signals spanning a range from zero to some very high value is not feasible as a practical matter. If, for example, it is assumed that a given transistor amplifier provides an amplification of to l, and that its maximum or saturation output voltage is l0 volts, then an input signal of .2 volt will produce an output signal of 5 volts. Similarly, input signals of .3 and .4 volt will produce output signals of 7.5 and l0 volts respectively. However, any higher input voltage, for example, .5 volt, will still only produce an output pulse of l0 volts since this is the saturation point of the amplifier. Referring to FIG. 4 the saturation level of the amplifier 29 has been arbitrarily represented by the vertical line SP. It will be observed that as the photomultiplier gain is progressively increased by connection to the taps T4, To, TS, more and more of the higher pulses representative ol decay events exceed the saturation point SP of the amplifier. Actually, those input pulses which do exceed the saturation point appear as amplifier output pulses all having un amplitude equal to the maximum amplitude obtainable from the amplifier. Thus, if the high voltage adjustment and the transducer gain are increased sutciently to malte the pulses in the lower portion ofthe spectrum greater in amplitude than very low amplitude noise pulses. and to get substantially all pulses in the spectrum above the lower level of discrimination, those pulses in the upper portion of the spectrum will tend all to have the same amplitude. so that they cannot be separated on the basis of amplitudes proportional to decay event energies. Under those conditions, background input pulses which exceed the amplitude of decay event pulses at the vupper end of the spectrum are not separable by a pulse height discriminator.

When working with low-activity radioactive sources or samples, and when background counts are appreciable, it is desirable to operate a spectrometer at or near optimum counting conditions, which serve to exclude a large proportion of background counts and to largely eliminate the undesirable etfects of shifts in the spectrum. Such optimum conditions exist when the pulse height analyzer is adjusted so that the etliciency of counting is high and the background counts are low, and more particularly when the ratio liz/B is near a maximum. This means that the window" A-B (FlG. 4) of the pulse height analyzer should be wide, yet not so wide that background pulses included in the window become great in number compared to the number of pulses originating from the isotope. Moreover, the window" should embrace the peak portion of the pulse height spectrum in order to malte the emciency as high as possible for a given window width.

A second part of optimum counting conditions is balance point operation," which reduces changes in eciency of counting which might otherwise occur due to drifts or changes in the gain of the system. This concept is made understandable by FIG. 5, where the pulse height spectrum for a given sample of a given isotope is illustrated by the curve 60. Those pulses which in amplitude fall between the window limits A and B are passed by the analyzer 29' (FIG. 2) to the scaler 30', assuming that the monitor channel produces coincident input pulses to the logic device 35.

Assume now that spectrum shifts slightly to the left, so that it is represented by the curve 60. Such a shift in the spectrum may occur, for example, as the result of undesired drifting or lowering in the gain of the photomultiplier, or as the result of color -or chemical quenching in the sample under test. In either 4instance the activity level of the isotope sample remains the same. When the spectrum 60', as compared to the spectrum ou, fewer pulses of amplitude greater than Vx will be re'-` Ceived and passed by the analyzer to the sealer; but more pulses of amplitude less than Vx will be received and passed. In other words, pulses lost from the spectrum shift are represented by the area 63, while pulses gained are represented by the area 66. The two areas are substantially equal by virtue of the fact that the center of the selected window A-B is adjusted to coincide approximately with the peak of the spectrum 60, i.e., so that a loss of pulses from a spectrum shift is balanced by a gain of pulses. Operation with the window so adjusted is thus termed balance point operation, and results in making the counting efficiency for a particular isotope substantially independent of minor, unavoidable drifts in the gain ot' the system. The balancing etiect occurs whether the spectrum shifts slightly to the litt or right. And from the foregoing explanation, it will be apparent that if the counting window is not so adjusted for balancepoint operation, shifts in the spectrum could result in appreciable erroneous changes in the counting efficiency and measured count rate.

snm-,satt

9 MULTI-LABEL AND MIXED SAMPLE MEASURE- MENTS ACCORDING TO THE PRESENT INVEN TIGN Thus far the environment of the invention has been de scribed with reference to the measurement of radioactivity in a source or sources having but a single isotope therein, and the reasons for coincidence monitoring, window adjustment for maximum operating conditions, including balance-point operation, have been explained. The present invention is concerned, however', with the independent measurement of the activity levels of two or more isotopes which are simultaneously present in a single source or sample, or which are present individually in mixed sources or samples. Merely by way of example, in radioactive tracer work a single sample may contain both H3 and C14, it being desired to measure individually the activity level of each of those two isotopes. While the invention may involve measurement of any number n of isotopes in multiple-label or mixed samples, or ow monitoring, and may be applied with advantage in gamma, alpha, and beta emission measurement, it will hereinafter be described by way of specific example in connection with measurement of double-label samples containing two (11:2) beta-emitting isotopes. Each Isotope I and II in a double-label sample will have a different characteristic energy spectrum for the decay events which it produces.

Referring to FIG. 6, there is illustrated a typical random distribution of pulses produced by a detector and transducer in response to decay events from two different isotopes (Isotope I pulses represented by the solid lines 69 and Isotope II pulses represented by the broken lines 70) present in a double-label source. As previously described, each isotope will undergo a plurality of decay events over a given period of time, the decay events being spread over a spectrum of energies. Thus, decay events in Isotope I will produce a few pulses 69a of slightly greater than zero amplitude and a few pulses 69C approxi mating a maximum amplitude Vmu-I for that isotope. However, the largest percentage of decay events occurring in Isotope I will produce pulses 6917 in the region of the average amplitude Vmg-I. Similarly, Isotope II will produce a few pulses 70a of approximately zero amplitude and a few pulses 70C approximating a maximum amplitude VWX-II for that isotope. The largest percentage of decay events occurring in Isotope lll will produce pulses 7Gb in the region of the average amplitude Vavg-II.

The spectrum of pulse heights for Isotope I at the amplifier output is represented by the curve 7l shown in FIG. 7, while the spectrum of pulse heights for Isotope II is represented by the curve 72. It is to be kept in mind that all of the pulses of FIG. 6 are present at the output of the transducer, so that the two spectra 71, 72 of FIG. 7 respectively represent two groups of pulses resulting from decay events from two respective Isotopes I and II.

As previously indicated, spurious events not associated with the radioactivity to be measured may produce bacicground and noise pulses at the output of the transducer. For example, cosmic rays striking the scintillator, despite shielding, may produce spurious light dashes; a contaminating isotope, such as K4 in the glass of a sample container may, to some degree, result in spurious light flashes which are transformed into voltage pulses. Regardless of their origin, such background pulses introduce into the count a degree of statistical uncertainty. Such background pulses appear throughout the range of pulse amplitudes. Therefore, in counting a particular isotope, it is desirable to limit the counting to a Window or discriminator band which includes only that portion of the isotope pulse spectrum occupied by the greater proportion of the isotope pulses, thus limiting the background count to those background pulses which happen to fall within the same amplitude band. As an approximation, background pulses have an amplitude distribution which is generally illustrated by the curve 74 (FIG. 8), the greater proportion'of such Ill pulses having amplitudes in the lower and higher pulse height ranges and a smaller proportion having amplitudes in the intermediate pulse height ranges. Finally, spurious electronic noise" signals generated in the preamplificrs and other electrical components produce low amplitude pulses, and it is impractical to attempt counting of pulses of an amplitude less than a threshold value arbitrarily represented at T.

Keeping in mind the foregoing considerations and with reference to FIG. 8, it will be apparent that rather severe limitations are placed upon the upper and lower limits which can be used for pulse height analyzing windows, particularly where balance-point operation and a generally maximumized ratio EZ/B are to be obtained. These limitations are even greater when two windows are to be used for analyzing two isotopes. For example, the saturation point SP of the electronic amplifier provides a positive upper limit for the window that is to analyze the most energetic isotope (i.e., Isotope II represented by curve '72). If the upper limit of a discrimination window is set slightly above the saturation point SP, then all amplified pulses exceeding the lower limit of the window will be passed, because none of the amplified pulses can exceed the SP amplitude. Similarly, the high spurious electronic noise level in the low amplitude pulse height range makes it impractical, if not impossible, to analyze pulses below a designated minimum voltage represented by the vertical line T. This places a minimum limit for the window that is to analyze the least energetic isotope (i.e., Isotope I represented by curve 7l).

In accordance with the present invention, the decay events of n isotopes (as here described, n=7) which each have a characteristic energy spectrum are converted into signals or pulses proportional in amplitude to the energies of the respective decay events. This may be accomplished by any suitable proportional type detector and transducer, the latter being illustrated in FIG. 9 as a liquid sciutillator mixed in a double-label sample 32 and disposed adjacent a photomultiplier 2d which is coupled to a preamplifier 26. The output pulses P-I from the preamplifier have the malte-up generally illustrated by FIG. 6 and comprise two spectra represented by curves 'Ill and 72 in FIG. 7.

Furthere-in accordance with the invention, the pulses so derived from n isotopes (here, two) are amplified to increase their amplitudes, thus spreading their amplitudes sufficiently to permit reliablediscrimination based on the relative heights thereof. However, as an important part of this step, the pulses are passed into n separate channels (here, two) and are linearly amplified by different factors or gains in each of the channels. As shown in the exemplary embodiment of FIG. 9, the pulses P-I are passed into two channels I and II which are in general similar to one another except for the fact that they include amplifiers which are adjustably set to provide two different gain factors. For this purpose, the channel I includes an amplifier Sl. comprised of an adjustable gain attenuator 79 and a fixed gain amplifier 80 producing output pulses P-2. Similarly, the channel II includes a variable gain amplifier fill* which is here shown as made up of a variable attenuator 79 and a fixed gain amplifier E0' producing output pulses P-3. Although channelI includes the adjustable attenuator 79 so as to have a variable gain amplifier 8l, this is not essential and a fixed gain amplifier may be used instead, as will be made clear below.

Before proceeding further with the generalized description of the invention, it will be helpful here to note riey one exemplary circuit which may be used for the variable attenuators 79 and 79. As shown in FIG. l0, a common emitter transistor 91 has its collector 9Ic coupled to the negative terminal 92 of a DC. supply source (not shown) and its emitter lie coupled to the positive terminal 94 of the D C. source. The base 9Ib of the transistor 91 is connected directly to one input terminal 95, the other input terminal 96 being Aconnected directly to .l .t a point 98 at ground potential. The transistor 91 forms an emitter-follower stage, its emitter Utile being connected through a capacitor C-l. to one end of a potentiometer 99 having its opposite end connected to the ground at 98. A potentiometer wiper 109 is selectively movable along the potentiometer 99 to tap a selected proportion of the voltage drop across the resistance when a pulse P-ll from the transducer is applied to the input terminals S35, 9d. Thus, the emitter follower 91 and thepotentiometer 99, 100 constitute an impedance-transforming variable gain amplifier which selectively attenuates the input pulses P-l according to the setting of the wiper M36'. The total gain at the amplifier 8l or 81' (FIG. 9) may thus be' adjusted over a wide range.

As the next step in practicing the invention, the pulses P2 and P3, amplilied by respectively different gains, are amplitude discriminated so that each channel l, ll passes a preselected amplitude band or window of pulse heights. As embodied in FIG. 9, each of the channels I, Il includes a pulse height analyzer 82, 82 which re ccivc the pulses P-Z and P-3, respectively. As shown in channel I, the analyzer 82 includes two discriminators 84A, 84B. both of which receive as inputs the pulses P-Z. Each of these discriminators is associated with a variable control voltage source which permits adjustment of the threshold amplitude which must be exceeded to ectuate or pass through the discriminator. For simplicity, such control voltage sources are illustrated as potentiometers 85d, 85b energized by batteries Ea, Eb.

T he discriminators 84A, 84B (and 84C, 84D) may be identical, and an exemplary one is shown in FIG. 1l as a monostable multivibrator 84A having a pair of common emitter amplifiers ltll, 1li?. connected across the negative and positive power supply terminals 92, 94, respectively. As previously indicated, means are provided for establishing a selectively variable discrimination threshold A for the multivibrator 84Asuch means here shown as comprising a voltage source Ea coupled to the base 102i: of amplifier 102 through a potentiometer 35a. The collector ltlitc of the amplilier is directly connected to the base 10211 of the amplifier 102 and one output terminal 104 of discriminator 84A, while the collector 1h21: of the amplifier 102 is coupled to the base 101i) of the amplifier 1(3-1 through capacitor C-Z. Thus, the monostabie multivibrator 84A has one stable state, i.e., with the amplifier 101 ON and the capacitor C-Z charged as shown; and one quasi-stable state, i.e., with the amplifier 102 ON.

To trigger the monostable multivibrator 84A, the base 102!) of the normally cut-oli amplifier 192 receives a pulse P-2 from the output terminal 1155 of the amplifier 8@ through a capacitor C-3. The arrangement is such that each negative output pulse P-2 from the amplifier Elli! drives the base lZb more negative in potential, thus tending to turn the multivibrator 54A ON so that the output terminal 104 is dropped in potential for a predetermined period (determined by the time constant of the circuit which discharges capacitor C-Z), thereby producing an output pulse PA. As the wiper of potentiometer 55a is selectively moved upwardly (FlG. ll), the bias potential at the base 102b is progressively made less negative and, consequently, a considerably higher amplitude pulse P2 is required to render the normally nonconductive amplitier 102 conductive and trigger the discriminator. Conversely, as the wiper of potentiometer 85a is moved downwardly (FIG. ll) the bias potential at the base i021? becomes more and more negative, thus decreasing the amplitude oF the pulse P-2 required to turn amplifier 1492 ON. it will be readily apparent from the foregoing clcscription that by selectively adjusting the potentiometers 85a, 35]; shown in FIG. 9 (and hence the potential levels at the base 102!) of each amplifier 162), an operator may adjust amplitudes which the pulses P-Z must exceed before those pulses are effective to trigger und pass through the discriminators 84A and 84B.

The pulse height analyzer d2 and the discriminators 86C, 84D therein are similar to those described for the analyzer 82. lt will sulhce to note, therefore, that by adjustments of the tour discriminators the amplitude levels A and B (channel l) and C and D (channel Il) may be individually set to different desired values, thus establishing the position and the width of two windows (FlG. l2) A-B and C-D, as described below.

To complete the measurement of the two Isotopes l and Il, the pulse height analyzers 32, S2' (FIG. 9) are caused to pass only the selected amplitude bands or windows of the pulses P-Z and P-S to appropriate indicating devices here shown as scalers 90, 90. For this purpose, the output pulses P-4- fromithe discriminator 64A are routed through a time delay device 1- as pulses P-S to one input of a normally closed gate 86 in a logic circuit 86. Assuming that a tirnecoincident pulse P-6 appears at the other input of the gate 86, the latter will produce an output pulse P-7 which is passed to one input of a normally open gate S9. If no pulse P-S appears at the other input of the gate 89 to close the latter, the pulse P-7 is passed to the sealer 90. It will be apparent, therefore, that any pulse P2 must exceed the amplitude setting A of the discriminator 84A before it can possibly reach the Scaler 99. On the other hand, if the same pulse P-2 exceeds the amplitude setting B of the discriminator MB, the resultant pulse from the latter will close the gate 89 and prevent that pulse P-2 from reaching the Scaler 90. Thus, only pulses P-2 Within the amplitude `band A B are passed tothe sealer 99.

The same operation obtains` in channel Il where the corresponding components permit passage of the pulses P-3 to the Scaler 90 only if the pulses P-3 have amplitudes between the voltages C and D established by the settings for discriminators 84C and 84D.

The purpose of the normally closed gates 8S and 88 is to provide coincidence monitoring, and thus to prevent darlt noise pulses originating in the photomultiplier and falling withinthe windows A-B and C-D from reaching the scalers 90 and 90. The monitoring channel M includes the second photornultiplier 24", the preamplilier 26, an amplilier S1 and a single base line d-iscn'rninator Sfr/fi'. The amplifier 81 may be a'fixed gain amplier, but has been shown as a variable gain aniplilier comprising an adjustable attenuator 79" and a xed gain amplifier 80". Although it is not essential, the gain of the amplifier 80 is preferably made equal to that oi the amplifier 8l, and the baseline at the discriminator 84A is made substantially equal to the base line A of the discriminator 84A. Thus, only amplified pulses P-lt) (resulting from monitor pulses P-Sl) which exceed the amplitude A will appear as pulses P- applied to the normally closed gates 88 and 88', and none of the pulses P-Z or P-3 `will reach the scale-rs 90 and 90' unless the monitor photomultiplier Z4" responds in timed coinci* dence with any pulse produced by the main photomultiplier 24.

Operation and Advantages Having thus described the principal steps and the apparatus embodying the invention, a brief summary with reference to the advantages obtained will be helpful. Referring to FIG. 12, let it be assumed lirst that the amplitude bands of the discriminators in channels I and Il (FIG. 9) have been set to define the respective windows A-B and C-D. Let it be assumed further that the amplifiers 3l and 81 have equal gains (contrary to the foregoing description). The pulses P2 and P3 in the twochannels will, under these conditions, have substantially identical amplitude distributions, and will comprise pulses in two spectra 7l, i2 (FlG. l2) resulting from the Isotopes l and ll. With the gain of the transducer adjustcd (by changing the setting of thc high voltage supply 25, lilG. 9) to locate the spectrum 72 substantially at balancepoint and with a good EZ/B ratio in the window CMD,

aliases 'ifi the Isotope II can be effectively counted in the channel Il. However, the spectrum 7l of the pulses from Isotope I involves pulses of such low amplitude ranges that there is total lack of balance-point operation, very low elciency and a poor EZ/B ratio for counting the Isotope I pulses in window A-B of the channel I. The base line A. cannot be lowered to good effect below the threshold point T because the noise pulses would then interfere with satisfactory counting. Even if the threshold noise prob lem could be eliminated, this mode of operation would require discrimination levels A and B so low that the triggering accuracy of the discriminators would be reduced.

Suppose that the gain of the transducer is increased (by adjusting the high voltage suply 25, FIG. 9) so that the spectrum of Isotope I pulses, which are included in the pulses P-2, is shifted to the position shown at 'lla in FIG.' 13. Isotope I pulses will now be measured in channel I substantially at balance-point conditions, with a good E/B ratio, and with a relatively few counts contributed by pulses resulting from Isotope II, the pulse spectrum for which is now represented by curve 72o. However, under these conditions, pulses P-3 scaled in channel II and originating from Isotope II include only those represented by the spectrum 72a between the C-D amplitudes. Thus, the counting `of Isotope II pulses in channel 76 now is far removed from balance-point conditions, has low eiiiciency, and a poor EZ/B ratio. This difficulty cannot be obviated to good advantage by increasing the discrimination level D because the C-D window would then exceed the saturation point SP of the amplifier 8l where no pulse height `discriinitiation is possible. Also, simply to eliminate the upper discrimination level D is not desirable because then all high amplitude background pulses would be counted.

From the foregoing discussion, it will be apparent that attempts to count Isotope II in channel II at approximately optimum conditions (FIG. l2) results in very poor counting conditions for Isotope I in channel I. If instead, adjustment is made to attempt counting of Isotope I at approximately optimum conditions in channel I, (FIG. 13) very poor counting conditions result for Isotope II in channel II. It is, in View of this, possible to change the high voltage setting for the photomultiplier so that neither of the Isotopes I or II is counted either at near-optimum or extremely poor conditions in the corresponding channel I or II; but this involves a high count of Isotope I in channel II and a -high count of Isotope II in channel I, thereby increasing the effective background in each channel and reducing statistical accuracy.

In accordance with the present invention, however, the amplifier gain in channel II intended to scale the higher energy Isotope II is made less than the gain of the other channel I. More specifically stated, the pulse spectrum for the lower energy Isotope I is adjusted for good or optimum counting conditions in a selected window for channel I, while the gain of the channel II is reduced relative to that of channel I to bring 'the pulse spectrum for the higher energy Isotope II into good or optimum counting conditions in a window C-D for channel II. Thus, in the present instance the attenuation factor of the attenuator '79 is increased to reduce the gain of the arnplilier 31' in channel Il. The two spectra of pulses which make up the pulses P-El will be shifted to those represented by curves llb and 72b in FIG. i4. Under these conditions, the pulses originating from Isotope `II will be scaled in channel II at balance point, with high eliiciency, with a high P/B ratio, and with relatively few or no pulses at all contributed by the lower energy Isotope I. Yet, under these conditions, the pulses P2 in channel I will be made up of pulse spectra 'ic and 72e (FIG. 14a) originating respectively from Isotopes I and II. The Isotope I pulses '71e may be scaled at optimum operating conditions, and with only a slight count contributed by pulses originating from Isotope II.

4It is to be emphasized that the selection of the location and width of the windows A-B and C-D is entirely independent since they are eliectivc on pulses P-Zt and P-3 which occupy entirely different spectra by virtue of the different amplifier gains in channels I and ll. By adjusting the relative gains of the two channels it would be possible to obtain the same results and advantages even though the windows A-B and C-D partially or wholly coincide as a strict matter of pulse amplitudes.

By following the invention as here described it is possible simultaneously to obtain statistically reliable counts for two or more isotopes present in a single radioactive source or sample. Because both isotopes may be measured 'with Igood or near-optimum operating conditions, it is totally unnecessary to subject a given double-label sample [to Vtwo separate counting periods and readjustment of the pulse spectra and windows between each of those two counting periods is obviated. Much time and labor is eliminated, `and more effective use: of the relatively expensive equipment is made possible.

To indicate in detail how the activity levels or" the two isotopes in a double-label sample can be obtained from one counting period, it may be observed first that the eiciency of counting the two isotopes in each of the two channels is determined .in the irst instance by placing a standard sample of Isotope I, having a known activity level, in the apparatus and observing the count rate in channels I and II. This yields the efficiencies S and T for Isotope I in channels I and II. Similarly a standard sample containing only Isotope II is counted and the eiiicioncies U and V for Isotope Ill in channels I and II are computed.

'Now a doubledabel sample having unknown activity levels a1 and a2 for Isotopes I and II is put through one counting period, and the count rates R1 and R2 in channels I and II are observed and become known. After subtracting background counts, the two channel count rates are made up of the :following components:

In channel I:

By solution of the simultaneous Equations l and 2, the

Thus, once 'the efficiencies or" counting the two isotopes in the two channels have been established, the individual levels of activity of the two isotopes in a double-label sample may be computed, on the basis of Equations 3 and l from Ithe observed count rates in channels I and II.

In the preferred form illustrated in FIG. 9, the two analyzing channels I and II, having different gains both reive pulses from the main photomu-ltiplier 2d. It would, of course, be feasible in 'the practice of the inven-tion to pass pulses from the two photomultipliers 24 and 24" respectively into the channels I and II, and to provide coincidence monitoring in each channel by pulses derived from the opposite photomultiplier by appropriate monitoring circuits.

It should be understood that while the invention has been described in connection with samples or sources containing two beta emitting isotopes, it will find equally advantageous use in measuring activity levels of gamma or alpha emitting isotopes. Moreover, the invention may he used effectively in measuring activity levels of two or more isotopes contained singly or in different combinations within dilicrcnt sources or samples, in view of the fact that us such mixed samples are placed successively in the apparatus, the particular isotope or isotopes therein will he counted with high clliciency in the corresponding channel or channels and with low eliciency -in the other channels-yet without any readjustment of the transducer gain, amplifier gains or window settings. Although individual samples have been discussed above, the invention may also be used in connection with monitoring `the activity level of a owing stream or a movable member carrying one or more of a plurality of isotopes, respective ones of which will be cfliciently counted in a corresponding one of a plurality of channels each having u different gain.

I claim as my invention:

l. For use in measuring the radioactive activity levels of one or more of n ditlerent radioactive isotopes contained singly or in different combinations within one or more sources, each of said isotopes having a different energy spectrum, the combination comprising means responsive to radioactive decay events occurring in a source for producing electrical signals proportional in amplitude to the energy of the corresponding decay events, n counting channels for receiving said signals, amplifying means in each or" said n counting channels for producing 'different signal gains in each channel, discriminator means in each of said n channels for passing only a selected amplitude hand of signals therethrough with the band of signals passed in each channel composed substantially of signals originating from decay events occurring in corresponding ditierent ones of said n isotopes, and indicating means for separately receiving the signals passed through each of said channels.

2. Spectrometer apparatus for measuring the activity levels of two different radioactive isotopes present in a source, with each isotope having a different characteristic energy spectrum, comprising, in combination, means responsive to radioactive decay events ourring in a sarnplc for producing electrical pulses proportional in amplitudc to the energy of the corresponding decay events, two pulse height analyzing channels for receiving said pulses, amplifying means in cach of said analyzing channels, means in one of said channels for adjusting the signal gain to a value different than the gain in the other of said channels, adjustable discriminator means in each of said channels for passing only a selected amplitude hand of the amplified signals therethrough, and counting means for separately counting the signals passed through cach of said channels.

3. Spectrometer apparatus for measuring the activity levels of n different radioactive beta emitting isotopes, each having a different energy spectrum, and contained in radioactive sources, comprising, in combination, detecting means responsive to beta emissions occurring in a source for producing electrical signals proportional in ampitude to the energy of the corresponding decay events, n counting channels for receiving said signals, means in each of said n channels for adjustably attenuating the input signals thereto, linear amplifying means in tandem with said attenuating means in each of said n counting channels, discriminator means in each of said n channels for passing only a selected amplitude band of,

signals therethrough with the band of signals passed in cach channel composed substantially or" signals resulting from beta decay events occurring in a corresponding different one of said rz isotopes, and counting means for separately counting the signals passed through each of said channels.

4. A liquid scintillation spectrometer for measuring the activity levcls of n radioactive isotopes disposed in a solvent containing n scintillator with each of said isotopes undergoing u plurality of different energy decay events resulting in the emission of radioactive energy and with the energy spectra o decay events for each said isotopes being different, comprising, in combination, photomultiplier means for converting light scintillations resulting from impingemcnt of radioactive particles with said scintillator into electrical pulses proportional in arnplitude to the energy or the decay events producing the particles, n linear amplifying means cach producing a different signal gain coupled to said photomultiplier means, n discriminator means respectively coupled to said n amplifier means for passing n amplitude bands of pulses with each band composed substantially of pulses resulting from decay events occurring in a different one of said n isotopes, and means for independently counting the pulses passed by each of said n discriminator means.

5. A liquid scintillationy spectrometer for measuring the radioactive strengths of n isotopes disposed in a solvent containing a scintillator with each of said istopes undergoing a plurality of different energy decay events resulting in the emission of beta particles and with the beta energy spectra of said isotopes being different, comprising, in combination, photomultiplier means for converting light scintillations resulting from impingement of beta particles with said scintillator to electrical pulses proportional in amplitude to the energy of the decay events producing the particles, n adjustable signal attenuating means coupled to said photomultiplier means, n amplilier means coupled to respective ones of said n attenuating means, n discriminator means respectively coupled to said n amplifier means for passing 1i amplitude bands of pulses with each band composed substantially of pulses resulting from beta decay events occurring in a different one of said n isotopes, and means for independently counting the pulses passed by each of said n discriminator means.

6. A liquid scintillation spectrometer for measuring the radioactive strengths of two isotopes disposed in a solvent containing a scintillator with each of said isotopes undergoing a plurality of different energy decay events resulting in the emission of radioactive particles and with the energy spectra of said isotopes being ditferent, comprising. in combination, photomultiplier means for converting light scintillations resulting from impingement of radioactive particles with said scintillator to electrical pulses proportional in amplitude to the energy of the decay events producing the particles, a source of high voltage coupled to said photomultiplier means, means for varying the high voltage supplied to said photomultiplier means so as to vary the signal gain produced thereby, two amplifiers each producing a different Signal gain coupled to said photomultiplier means, two discriminator means respectively coupled to said amplifiers for passing two amplitude bands of pulses with each band composed substantially of pulses resulting from decay events occurring in a different one of said two isotopes, and means for independently counting the pulses passed by each of said discrirninator means.

7. In spectrometer apparatus for successively measuring the radioactive strengths of samples singly containing different ones of n isotopes with each of said n isotopes undergoing a plurality of ditterent energy decay events resulting in the emission of a plurality of radioactive particles and with each isotope being uniquely identified by its energy spectrum, the combination comprising, means for converting the energy of the decay events in a sample into light energy, means for converting the light energy produced into electrical pulses proportional in amplitude to the energy of the corresponding decay events, n analyzing channels for simultaneously receiving said pulses, means for independently attenuating the input pulses to each of said channels so that any given input pulse produces a different amplitude input to each channel, amplifying means in each of said channels. discriminator means in each of said channels for passing only a preselected amplitude band of pulses therethrough, and means for separately counting the pulses passed through each of said n channels, all of said discriminator means and all of said attenuating means being preset so that each of said n isotopes are counted at relatively high efiiciency in a different corresponding one ot' said n channels and at low etiiciency in the other of said channels.

8. In spectrometer apparatus for measuring the activity levels of sources each containing at least one of two isotopes with each of said isotopes undergoing a plurality of different energy decay events resulting in the emission of a plurality of beta particles and with each isotope being uniquely identified by its energy spectrum, the combination comprising, means for converting the energy of each beta decay event into light energy, means for converting the light energy produced into electrical pulses proportional in amplitude to the energy of the corresponding decay events, a pair of analyzing channels for simultaneously receiving said pulses, means for independently attenuating the input pulses to each of said channels so that any given input pulse produces a different amplitude input to each channel, amplifying means in each of said channels, discriminator means in each of said channels for passing only a preselected amplitude band of pulses therethrough, and means for separately counting the pulses passed through each of said channels, said discriminator means and said attenuating means being preset so that each of said isotopes are counted at high eliiciency in a different corresponding one o said channels.

9. Spectrometer apparatus for measuring the activity levels of two different radioactive isotopes singly and doubly contained in diterent samples with each of said isotopes undergoing a plurality of different energy decay events resulting in the emission of a plurality of radioactive particles and with each isotope having a dillerent characteristic energy spectrum comprising, in combination, scintillator means for converting the energy of each decay event into light energy. a pair of photomultipliers disposed adjacent to said scintillator means for converting the light energy produced into electrical pulses proportional in amplitude to the energy of the corresponding decay event, first and second pulse channels coupled to one of said photomultipliers to receive the pulses therefrom, a third pulse channel coupled to the other of said photomultipliers. amplifying means in each of said channels with the signal gain produced thereby in one ot' said channels bcing different from the gain in the other two channels, a first analyzer connected to receive the amplified pulses in said one channel. a second analyzer connected to receive the amplified pulses in one of said other two channels, said analyzers each including means for passing only a preselected amplitude band of pulses therethrough, a first logic circuit coupled to and receiving input signals from said iirst analyzer, a second logic circuit coupled to and receiving input signals from said second analyzer, each of said first and second logic circuits coupled to and receiving coincidence control input signals from a channel which derives its pulses from that one of said photomultipliers other than the photomultiplier which originates the pulses passed by the corresponding analyzer, and first and second counting devices respectively coupled to said first and second logic circuits, said logic circuits each including means for passing pulses to the associated counting device only when that logic circuit simultaneously receives an input signal and a coincidence control input signal.

l0, Spectrometer apparatus for measuring the activity levels of two different isotopes contained in a liquid sample with each of said isotopel undergoing a plurality of different energy decay events resulting in the emission oi' a plurality of radioactive particles and with each isotope being uniquely identilied by its energy spectrum, comprising, in combination, scintillator means in said sample for converting the energy decay events into light flashes, a pair of photomultipliers disposed adjacent to said sample for converting said light flashes into electrical pulses proportional in amplitude to the energy of the corresponding decay event, first and second analyzing channels coupled to one of said photomultipliers, a monitoring channel coupled to the other ot said photomultipliers, amplifying means in each of said analyzing channels, selectively adinstable control means in one of said analyzing channels for varying the signal gain therein and making it different than the gain in the other analyzing channel, discriminator means in each of said analyzing channels for passing only a preselected amplitude band of pulses therethrough, a first logic circuit coupled to and receiving input pulses from the discriminator means associated with the one of said analyzingV channels, a second logic circuit coupled to and receiving input pulses from the discriminator means associated with other one of said two analyzing channels, each of said tirst and second logic circuits coupled to and receiving coincidence control input signals from said monitoring channel and including means for passing an input pulse only when it coincides in time with a control input signal, and first and second counting devices respectively coupled to receive pulses from said rst and second logic circuits.

l1. Spectrometer apparatus for measuring the activity levels of two different radioactive isotopes singly and doubly contained in dilierent samples with each of said isotopes undergoing a plurality of different energy decay events resulting in the emission of a plurality ot' radioactive particles and with each isotope a dierent characteristic energy spectrum, comprising, in combination, scintillator means for linearly converting the energy of decay events in a sample into light flashes, lirst and second photomultipliers disposed adjacent to said scintillator means for converting said light dashes into electrical pulses proportional in amplitude to the energy of the lcorresponding decay event, irst and second analyzing channels coupled to said lirst photomultiplier, a third monitor channel coupled to said second photomultiplier, attenuating means in one of said analyzing channels, amplifying means in each of said analyzing channels, means for adjusting `said attenuating means to selectively establish ditierent gains in said first and second channels, adjustable discriminator means in each of said two analyzing channels for passing only a preselected amplitude band of pulses therethrough, a first logic circuit coupled to and receiving input pulses from the discriminator means associated with said first analyzing channel, a second logic circuit coupled to and receiving input pulses from the discriminator means associated with said second. analyzing channel, means connecting said monitor channel to transmit pulses therein as coincidence control input signals to said irst and second logic circuits, said first and second logic circuits each including means for passing a pulse only when an input pulse from the associated discriminator means coincides in time with a control signal from the monitor channel, and first and second counting devices respectively coupled to said first and second logicl circuits.

12. The method of measuring the activity levels of one or more different radioactive isotopes contained in one or more sources, each of said isotopes having a characteristic energy spectrum of the decay events occurring therein, said method comprising the steps ot' converting the radiant energy for substantially all decay events into electrical pulses substantially proportional in amplitude to the corresponding decay events, simultaneously applying each electrical pulse to a plurality ol channels, independently amplifying by dilierent gain factors: the pulses in each channel, discriminating said amplified pulses in each channel so as to pass only preselected, amplitude bands of pulses through respective ones of said channels while rejecting pulses outside such amplitude bands, and separately determining the rate of occurrence of the discriminated pulses in each ot said channels.

i3. The method of measuring the activity levels of two isotopes contained in a single source, each of said isotopes aliases having a different characteristic energy Spectrum of the decay events occurring therein, said method comprising the steps of converting the radiant energy from substantially all decay events occurring in the source into electrical pulses proportional in amplitude to the energy of the corresponding decay events, simultaneously applying each electrical pulse to a pair of channels, simultaneously adjusting the amplitude of input pulses to each channel to adjust the pulse amplitude spectra, independently attenuating the input pulses to one of said channels to separate the amplitude spectra of such pulses from the amplitude spectra of input pulses to the other channel, amplifying the adjusted and attenuated pulses in each of said respective channels, discriminating said amplied pulses in each channel so as to pass two separated amplitude bands of pulses through respective ones of said channels, each of said bands being substantially composed of pulses resulting from decay events occurring in dilerent ones of said isotopes, and separately counting the number of discriminated pulses passed through each channel.

14. The method of determining the activity levels of two isotopes each of which has a different characteristic spectrum of energy of decay events occurring therein, said method comprising the steps of converting decay events occurring in the isotopes into electrical pulses of amplitudes proportional to the energies of the corresponding decay events, routing all of said pulses into two separate channels, amplifying the pulses in each of said channels,

channel so that the height spectrum of pulses originating from decay events in the second isotope is at balance point between said C and D limits, and separately determining the rate of occurrence of the discriminated pulses in the two channels.

References Cited in the lile of this patent UNTTED STATES PATENTS 2,837,659 Hendee et al. lune 3, 1958 2,943,199 Konneker June 28, 1.960 2,957,989 Hull Oct. 25,-1960 3,004,167 Owen Qct. l0, 1961 OTHER REFERENCES An Automatic Brain Scanner, oy Reid et al., from International Journal of Applied Radiation and Isotopes, vol. 3, pages l to 7; 1958, published by Pergamon Press Ltd., London. 

8. IN SPECTROMETER APPARATUS FOR MEASURING THE ACTIVITY LEVELS OF SOURCES EACH CONTAINING AT LEAST ONE OF TWO ISOTOPES WITH EACH OF SAID ISOTOPES UNDERGOING A PLURALITY OF DIFFERENT ENERGY DECAY EVENTS RESULTING IN THE EMISSION OF A PLURALITY OF BETA PARTICLES AND WITH EACH ISOTOPE BEING UNIQUELY IDENTIFIED BY ITS ENERGY SPECTRUM, THE COMBINATION COMPRISING, MEANS FOR CONVERTING THE ENERGY OF EACH BETA DECAY EVENT INTO LIGHT ENERGY, MEANS FOR CONVERTING THE LIGHT ENERGY PRODUCED INTO ELECTRICAL PULSES PROPORTIONAL IN AMPLITUDE TO THE ENERGY OF THE CORRESPONDING DECAY EVENTS, A PAIR OF ANALYZING CHANNELS FOR SIMULTANEOUSLY RECEIVING SAID PULSES, MEANS FOR INDEPEND- 